As a die-cushion controlling device for controlling elevating operations of a die-cushion pad driven by a servo motor, a die-cushion controlling device proposed in, for instance, Patent Document 1 has been conventionally known. According to the die-cushion controlling device disclosed in Patent Document 1, a cushion stroke of a die cushion is controlled by a position control until an upper die of a slide contacts a die-cushion pad with a workpiece sandwiched therebetween. On detecting a change in a current of a servo motor when loading starts to be applied on the die-cushion pad, the control is switched from the position control to a pressure control by a detection signal of the current change, so that a preset cushion pressure is applied on the die-cushion pad. Since such a die-cushion controlling device as arranged above can switch the control from the position control to the pressure control, drawing can be favorably conducted.
In a die-cushion controlling device in which a position control and a pressure control are switchable, a control is switched from the pressure control to the position control again when a slide has reached a bottom dead center, so that a die-cushion pad is moved up within a predetermined time after an upward movement of the slide is started so as to return to a stand-by position for the next processing. In addition, when the slide has reached the bottom dead center position, the die-cushion pad is controlled to stop at its own bottom dead center position for a predetermined time (bottom dead center locking) in order to assure that a drawing is reliably conducted to the end.
[Patent Document 1] JP-A-10-202327 (page 3)